She IS a whore. A woman who sells sex for money. By calling her a HORSEFACE, the sound signifier "HOR" is planted into many minds. Whether they realize it or not, the sound will play in their minds and invoke the associated word, spelled WHORE. He knows exactly what he is doing. Horseface isn't legally actionable and while "unpresidential" and "undignified", he is dishing out a measured dose of what they have been dishing onto him. It's just. Fight fire with fire.